1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine or a printer for transferring a toner image formed on an image bearing member, for example, by an electrophotographic process to a recording material, and thereafter fixing the toner image to thereby obtain a permanent image on the recording material.
2. Description of Related Art
As a color image forming apparatus capable of outputting a full-color image, there has heretofore been put into practical use one having a construction in which at a first transferring region, formed in the portion of contact between the surface of an image bearing member and the surface of a transfer material carrying member, a first transferring bias is applied to a first transfer member disposed on the back of the transfer material carrying member to thereby once transfer, i.e., primary-transfer, a toner image on the surface of the image bearing member to the surface of the transfer material carrying member. Then, the transfer material is passed to a second transferring region, formed in the portion of contact between the transfer material carrying member and a second transfer member, and a second transferring bias is applied to thereby again transfer, i.e., secondary-transfer, the toner image on the surface of the transfer material carrying member to the transfer material.
In the above-described image forming apparatus, color reproduction is effected with toners of four colors, i.e., yellow, cyan, magenta and black, superimposed one upon another. Therefore, unless the density of the toner images of the four colors is accurately adjusted, good color balance cannot be obtained.
Accordingly, in many color image forming apparatuses, there is carried an image density control mechanism for automatically adjusting image forming conditions such as charging potential, an exposure amount and a developing bias. A popular method for this image density control is as follows.
First, a predetermined image for density control (hereinafter referred to as the patch) is formed on the image bearing member or the transfer material carrying member, and the density of the toner image is detected by an optical sensor (density sensor) comprising a light emitting element and a light receiving element. The image forming conditions are then adjusted in conformity with the detected density of the toner image.
FIG. 2 of the accompanying drawings is an example of a schematic view of the above-mentioned patch for density detection. In FIG. 2, Y1-Y4 are test patches for detection when a developing bias for yellow was set to four stages, i.e., −100V, −150V, −200V and −250V, and density was changed. Each of these patches is of a size of 2 cm square. Likewise, M1-M4 are test patches for the detection of magenta, C1-C4 are test patches for the detection of cyan, and K1-K4 are test patches for the detection of black. The patches for density detection are formed so as not to overlap one another, and the arrow in FIG. 2 indicates the direction of movement on the image bearing member or the transfer material carrying member.
Discretely from the above-described density detection, in such an image forming apparatus, in order to further improve the quality of the final image, a minute toner image of a dot-type (a shock band preventing pattern) formed by a yellow toner or the like can be additionally formed on the image bearing member, besides a toner image of an image pattern desired by a user (see, for example, Japanese Patent Application Laid-Open No. H11-052758).
This is because when a toner image formed on the surface of the image bearing member is primary transferred to the surface of the transfer material carrying member (intermediate transfer belt), a minute fluctuation of rotation sometimes occurs to the image bearing member and this may cause uneven exposure to a laser beam. In such a case, an image streak occurs to a toner image subsequently formed on the surface of the image bearing member. In order to prevent the occurrence of such an image streak, the minute toner image is formed.
A dot toner image pattern of an arrangement shown, for example, in FIGS. 3 and 4 of the accompanying drawings is formed as a dot toner image pattern. A box in these figures represents 600 dpi, and the data of pixels indicated by black in the figures is defined as FFh, whereby a minute dot toner image is formed at the relevant position.
The dot toner image pattern shown in FIG. 3 comprises dot toner images of the size of a pixel arranged at intervals of 0.46 mm in each of a main scanning direction (the rotational direction of the image bearing member) and a sub-scanning direction (the rotational direction of the transfer material carrying member).
Also, FIG. 4 shows dot toner images of the same size arranged at an oblique angle of 45° with respect to the main scanning direction, and the dot interval in the sub-scanning direction is 0.34 mm. The toner image by any one of these patterns is formed in overlapping relationship with the entire area of a toner image of an image pattern desired by the user.
The average printing rate when such a dot toner image is formed on a photosensitive drum as the image bearing member differs in its appropriate value from one image forming apparatus to another, depending on the contacting force of a primary transfer roller with the photosensitive drum, the difference in surface peripheral speed between the photosensitive drum and the intermediate transfer belt as the transfer material carrying member, etc. But, in such an image forming apparatus, design is made such that the printing rate is of the order of 0.05-1% relative to the toner printing rate of a solid image portion of each color on the photosensitive drum. This is because when the printing rate is too low, the fluctuation of the rotation of the photosensitive drum cannot be suppressed, and when the printing rate is too high, a level which can be visually confirmed by the user results.
Further, a conventional image forming apparatus has the feature that there is formed a pattern (a forgery discriminating pattern) representative of the follow-up information of the image forming apparatus such as, for example, the manufacturing number, manufacturer and date of manufacture of the image forming apparatus (see, for example, Japanese Patent Application Laid-Open No. H11-41445).
According to such a construction, the aforementioned image streak can be prevented. On the other hand, when a bill, a negotiable instrument or the like has been forged by the use of an image forming apparatus, the dot size or arrangement of dot toner images formed on the forged matter can be researched to thereby specify the image forming apparatus. In such a manner, it becomes possible to obviate the forgery of a bill, a negotiable instrument or the like.
FIG. 5 of the accompanying drawings shows an example of the pattern of the dot toner images. A box in the figure represents 600 dpi, and the data of pixels indicated by black in the figure is defined as FFh, whereby a minute dot toner image is formed at the relevant position. In this pattern, a dot toner image of a size of 1 pixel (main scanning direction)×4 pixels (sub-scanning direction) forms a pattern representative of the follow-up information of the image forming apparatus. Also, this dot toner image is formed in overlapping relationship with the entire area of a toner image of an image pattern on a bill, a negotiable instrument or the like.
However, the final image obtained by the above-described image forming apparatus causes the following inconvenience of image density.
In the above-described conventional image forming apparatus, when density control is effected, the above-mentioned shock band preventing pattern or forgery discriminating pattern is not superposed on each of the yellow, magenta, cyan and black patches. On the other hand, when an actual image is formed, the image is formed with a pattern superposed thereon and therefore, image density has sometimes been fluctuated. The cause of the problem of this fluctuation of image density will hereinafter be described.
The above-mentioned shock band preventing pattern and forgery discriminating pattern generally use the yellow color, and if the above-described pattern is imprinted on the entire surface, it is difficult to see by the human eyes, but it affects the density of an actual toner image (the density becomes high), and particularly in a high light portion (low density portion), noise and the yellowishness of the texture become conspicuous. Also, the above-described pattern is singly difficult to see, but it will sometimes be visualized if it is mixed with a toner of other color by subtractive color mixture.
Even if in order to obtain an optimum quality of image, the above-described density control is effected to thereby correct image density, the above-described shock band preventing pattern and forgery discriminating pattern overlap the actual toner image over the entire area thereof. If at this time, the toner density is high, there is little or no contribution by the above-described pattern overlapping the actual toner image. But, when the toner density is low, that is, in the high light portion, the contribution of the above-described pattern becomes great, and the difference between the density obtained by the result of the density control and the density of the actual toner image is remarkably seen.